DESCRIPTIVE VERSUS HOMOLOGOUS TERMINOLOGY: DESCRIBING THE CRINOID

Descriptive morphology based on a firm foundation of homology is essential for phylogenetic analysis, and the disconnect between descriptive terms and homology has been a major obstacle for deciphering phylogenetic relationships among echinoderms. Recent studies using the Universal Elemental Homology schema have been instrumental in deciphering relationships among echinoderm classes. Although less problematic within the Crinoidea than with the Echinodermata as a whole, homology issues exist among crinoids. Crinoids have a multi-element mesodermal skeleton that may contain a few 10s to a few 100,000s of individual plates, and homoplasy in form, if not function, is a confounding issue among crinoids.

In this paper we open a dialog to address issues of crinoid morphological terminology in the hope of developing a working consensus. The following are questions addressed in this work. What is the first anal plate on a disparid crinoid? What defines an arm? Are the arms on the Catillocrinidae arms? What is a pinnule versus a ramule, really? Do arms with “pinnular units” have true pinnules? What should be the name of fixed plates between and within rays? Nodals, nudinodals, and internodals – where are the homologies? Is it a cirri or a rhizoid? Here we discuss instances where conflict may exist between descriptive morphology and homology.